The present invention relates to an improved process for removing nitrogen impurities, by means of an aqueous acid solution, from a liquid hydrocarbon mixture. More particularly, the present invention relates to the treatment of a liquid hydrocarbon mixture containing unsaturated hydrocarbons with an aqueous acid solution.
Generally, in order to remove aromatic compounds therefrom, liquid hydrocarbon mixtures are subjected to treatments such as hydrogenation or reforming. However, it is well known that the catalysts used in the hydrogenation treatments, which are most frequently a noble metal on a support, or the catalysts used in the reforming, are poisoned by the nitrogen compounds which are present in the liquid hydrocarbon mixtures. One of the required conditions for subjecting a hydrocarbon feed to a reforming process, is a nitrogen compound content lower than 0.5 ppm. Indeed, the presence of nitrogen compounds in a hydrocarbon feed which is to be reformed gives rise to the formation of NH.sub.3 which may be either adsorbed at the catalyst level, with the resulting neutralization of the acid sites, or recombined with HCl formed during the reforming, with formation of a salt which forms a deposit on the apparatus.
For the most part, the nitrogen compounds contained in the hydrocarbon feeds are present in the form of basic nitrogen compounds. In order for the hydrocarbon mixtures to be suitable for reforming or for subjecting to a hydrogenation treatment in the presence of a noble metal catalyst, the basic nitrogen compound content must be reduced to less than 2 ppm and preferably to less than 1 ppm. The rest of the nitrogen compounds, inasmuch as their content does not exceed 10 ppm, may readily be eliminated by hydrogenation under operating conditions similar to those of a desulfurization.
If the basic nitrogen compound content is not reduced to less than 2 ppm, the residual basic nitrogen compounds will have to be eliminated by hydrogenation under severe conditions, and particularly under a high hydrogen partial pressure. Such conditions are difficult to use in conventional refineries.
In order to avoid these drawbacks, it has been proposed to treat with an organic or inorganic acid those hydrocarbon feeds which have a boiling point of from 200.degree. to 600.degree. C., and which have to be further submitted to a thermal or catalytic cracking to form gasolines. However, the nitrogen compounds are only partially removed by this acid treatment, and therefore, when the hydrocarbon feed is cracked, nitrogen compounds are still formed, and these have to be removed before carrying out the reforming. Moreover the operating conditions of the acid treatment of the hydrocarbon feed before cracking are not suitable for use with the cracked products.
Indeed, the cracked products contain unsaturated hydrocarbons which can polymerize if they are submitted to an acid treatment under the conditions suggested for the acid treatment before cracking. The polymerization of these unsaturated hydrocarbons depends on the acid concentration of the solution and on the contact time of the feed with the acid solution. The conditions of the acid treatment of the feed before cracking are an acid concentration of from 85 to 100% and contact times of from 5 minutes to 2 hours. Under such conditions, the unsaturated hydrocarbon present in the feed will polymerize.
It has been proposed to directly treat the cracked products with an acid solution, particularly a sulfuric acid solution, but under specific conditions of acid concentration and contact time. Thus it has been proposed in "The Science of Petroleum", Vol. III, page 1773 to use an aqueous sulfuric acid solution, having an acid concentration of from 40 to 98% with contact times such that the lower the acid concentration, the higher the contact time. Nevertheless, the polymerization of the unsaturated hydrocarbons cannot be completely avoided, even if the acid concentration is as low as 40%.
In order to reduce the contact time, and therefore the polymerization rate, it has been proposed to use more concentrated sulfuric acid solutions, but using certain specific methods of contacting the feed with the sulfuric acid solution. Examples of contacting devices, which have been suggested are mixer batteries wherein the feed and the concentrated sulfuric acid solution move in countercurrent flow, and a contacting column having a particular contact bed described in the U.S. Pat. No. 2,999,807. According to the process described in this U.S. patent, one uses an aqueous sulfuric acid solution having an acid concentration of at least 65% together with contact times in the column of from 3 to 25 seconds. This column includes a contact bed made of an inert material which may be a ceramic, coke or glass having a hydrophilic surface so that it is easily wetted by the sulfuric acid which is thereby retained on the surface, with formation of a large contact surface. However, this process does not avoid the formation of a significant amount of polymerized products.
Another process for acid treatment of hydrocarbons is disclosed in U.S. Pat. No. 3,177,137 in which hydrocarbon droplets are dispersed in a strong acid, such as 98% sulfuric acid, and a supernatant layer of clear, acid-free hydrocarbon is formed above the dispersion. Spent acid may be recycled, but the amount must be restricted to produce a quality product. Large amounts of acid are consumed in the process.
U.S. Pat. No. 3,719,587 discloses a process for treating coal-derived naptha by purging with an inert gas and washing the hydrocarbon by passing it together with water or a dilute acid through a centrifugal pump or orifice mixer and then allowing the resulting mixture to separate in a settling zone. If the amount of treated feed and the amount of used aqueous solution which must be disposed of are taken into account, it is apparent that not only the amount of consumed acid is not negligible, but also the amount of used aqueous solution to be neutralized before disposal requires the use of suitable purification plants.